OCR tool+ to autogen a suggested alt text. The path of least resistance needs to be lowered.

Alternatively, inverting the paradigm is likely to cause less issues and push back. Add the automated tool the the end user in need of the version. This obviously creates the issue of data quality and trust, but for the smaller group. What if there was a reply field silently posted to everyone’s notifications feed indicating anonymous instances of the tool being used to fill in the gaps for alt text? The message would need to be opt out or carefully presented. Perhaps it could be possible to modify the post itself via the tool? Better yet, make the alt text field a Wikipedia style affair anyone with an account can edit, but with a lock available to the OP. That would create much more healthy awareness of the need for alt text, as people posting the content will see the places where gaps are filled by an automated tool. It gives them the chance to edit. This does little to initially improve the experience of the most active alt text users, but it creates a strong cultural shift in awareness that should improve the situation greatly in the long term IMO.